1. Field of the Invention
The present invention relates generally to power meters, and more specifically to means to ensure power meter accuracy, and still more particularly to a method and apparatus that eliminates the need to calibrate or zero a power meter.
2. Discussion of Related Art Including Information Disclosed under 37 CFR §§1.97, 1.98
For over 50 years all RF and microwave power meters have provided a means to zero and calibrate the meter before use. For bench use, the need to zero and calibrate is a source of inconvenience and irritation. It can also be a source of error. This requirement to zero and calibrate a power sensor becomes more problematic in large systems. In racked systems the need to zero and calibrate contributes uncertainty, adds hardware and engineering, and increases software complexity and cost.
The use model typical for power meters and sensors is: (1) remove the power sensor from all RF and microwave power sources; (2) zero the meter; (3) if required, connect the power sensor to a calibrated source; (4) if required, calibrate the power sensor; and (5) remove the power sensor from the calibrator. After taking these measures, the power meter is purportedly ready for use, and it is connected to a measurement port.
Some newer sensors introduced internal switches allowing an internal (to the sensor) load and reference to be switched in or out for purposes of zeroing and calibration. And while this approach mitigates some problems it causes others. The problems mitigated are the need for the user to make/break electrical connections during calibration and zeroing. However, its very nature introduces additional expense. These switches introduce additional uncertainty. In addition, the process of switching induces discontinuities in the signal paths. These discontinuities can be the source of unwanted reflections especially in systems with sensitive receivers.
Calibration before use should not be confused with annual calibration. Calibration before use (if required) is accomplished by connecting the power sensor to a calibrated reference after it has been zeroed. This source is a part of the power meter. It is most commonly a 1 mW, 50 MHz source. Zeroing or calibration before use is typically recommended when ambient temperature changes by about 5° C.
To understand how zeroing and calibration affects system hardware and software design, it is useful to consider a simple example. Assume a modest automated system with a number of paths, such as the system 100 one shown in FIG. 1, which includes an input (start) path 110, a number of circuit elements and devices 120, and an output (end) path 130. All measurements using these paths must be path loss compensated. Designers of automated systems want to minimize user interaction and test execution time.
In practical systems, path loss requires periodic measurement. Path loss is generally determined by taking the difference between two measurements. This implies that one measurement must be taken at points proximate each end of the path, and these points must be accessible to the measurement device. The low cost and relative accuracy of power meters make them a good choice for path loss measurements.
However, entirely automated path loss measurement presents complications, many of which are related to the need to zero and calibrate the power meter/sensor.
The following sets out a path loss calibration algorithm which requires some user interventions and is not concerned with minimizing user interaction or speeding up overall measurements:
User removes sensor from all system connections                User zeros the sensor        If calibration is required                    User connects sensor to reference            Calibrates the sensor                        End if        User connects sensor to input measurement point        For each frequency                    Make measurement                        Next Frequency        User connects sensor to the output measurement point        For each frequency                    Make measurement                        Next Frequency        For each frequency                    Apply difference between input and output to path correction            Apply systematic or known offsets to path correction                        Next Frequency        
In newer sensors with internal switches, the points requiring the user to remove and/or connect the sensor in the process outlined above would be replaced with commanding the sensor to switch in the load or reference as needed. Clearly this is an advantage in terms of speed and allows the system to be more automated. However, there is a clear trade off in terms of additional expense and the discontinuity issues and changes in system match are still present.
This is a simplified path calibration process showing all user interventions in bold typeface. What is not noted is the need for the user to pay close attention to the proximity and routing of signals and the placement of the power meter and sensor in the system. This will be dealt with later in this disclosure.
A completely automated process is far more desirable. However, because power meters require zeroing and/or calibration, complete automation is more expensive and more difficult to achieve. The inventive automated method and apparatus does not depart substantially from the user-mediated process outlined above. However, the design of the inventive system is modified such that the system breaks the sensor connection to the system and then makes the connections to the sensor reference, and input and output measurement points. These connections are managed through switches.
Consider the first user interaction from the sequence outlined above: User removes sensor from all system connections: For presently available sensitive sensors (sensitivities below −65 dBm) the sensor is often connected to a port that guarantees signal levels lower that −80 dBm. This can be done by connecting the power sensor to a port terminated into a 50Ω load. This sometimes requires the addition of a microwave switch. This switch must be managed by software and the additional uncertainty and correction must be accounted for in the measurement.
What is generally not recognized is the need to keep track of the last zero and calibration in the system software. On complex systems this must be done across many tests and measurements. The alternative to doing this is to zero and calibrate before each major test. This causes extra wear on switches and can be significant in some case and costs extra measurement time since zeroing and calibration (in relative terms) a time-consuming measurement.
The next user interaction is even more problematic: User connects sensor to reference. The power meter sensor is specified based on the assumption that during calibration the sensor is connected directly to the calibration source. Some manufacturers recognize the significance of this problem and have included references internal to the sensors. It should be clear that the calibration reference must be routed to every sensor or every measurement point must be routed to a single sensor. As a result there will be more switches, greater uncertainty and greater cost.
There is one other, perhaps more subtle, problem associated with calibration: The existence of the calibrator and the need to make it extremely accurate forces the manufacturer of the power meter to fix the connector type and sex. This decision is not without negative impact on the system.
By fixing the type and sex of the connector of the calibrator, the manufacturer also fixes the type and sex of the power sensor. This almost always forces the system designer to employ adapters between the system and the power meter. Every adaptor adds further uncertainty to the measurements of the system. This small initial uncertainty may have an unexpectedly larger impact because it is seen in subsequent uncertainty calculations.
The next user interaction is: User connects sensor to input measurement point. Again, this intervention poses additional problems. In most RF and microwave systems the measurement should be as close as possible to the components being measured. In this case the paths are likely buried inside a complex system (path routing accomplished with a switch matrix). The paths within the system are generally not accessible. The only alternative in most systems is to bring the measurement port out to the power sensor.
This can add significant path length to signals that may not be able to tolerate the associated loss or mismatch. This poses serious problems for system developers. The result is often serious compromises in accuracy test time and it adds complexity and hardware costs.
As a practical matter the relationship of zeroing and calibration to temperature can be shown to be almost exclusively associated with the sensor (or detector) itself. In other words, the meters have traditionally had little impact on zero and calibration drift issues. This can be demonstrated using the venerable Agilent Technologies, Inc HP432A and HP478A in their normal configurations. Experiments show that an HP478A (connected to an HP432A) in a normal environment will require coarse and fine zeroing after being turned on.
During roughly the first half hour after it has been turned on, the HP478A will need to have this process repeated about every 4-5 minutes. It may require zeroing more often if the most sensitive ranges are used. As the device stabilizes it still requires a zeroing and calibration every half hour, and more often if the environment is less well controlled.
In addition, zero and calibration will be required more frequently if the device is handled. Simply picking up a newly zeroed device will cause significant drift of the zero on the lower ranges. This same routine can be repeated with other sensors and RF detectors.
On the other hand, if the HP478A power sensor is placed in a temperature controlled environment, it can remain zeroed indefinitely. Experiments show that an HP478A that is heater stabilized temperature to within ±½° C. will remain zeroed indefinitely. In experiments conducted by the present inventors, however, only the HP478A was temperature stabilized; the HP432A power meter was not temperature stabilized. During this time ambient temperature ranged from about 50 F to more than 80 F.
Accordingly, despite the fact that awareness of the problem and its source has existed for more than fifty years, power meters have remained fundamentally unchanged.
The foregoing systems reflect the current state of the art of which the present inventors are aware. Reference to, and discussion of, these systems is intended to aid in discharging Applicants' acknowledged duties of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the prior art methods or devices disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.